Overview
Ti-6Al-4V (Grade 5) is the workhorse titanium alloy — roughly half of all titanium produced worldwide is this single alloy. It’s a two-phase alpha+beta material: 6% aluminum stabilizes the alpha phase (HCP, strong but less ductile), 4% vanadium stabilizes the beta phase (BCC, more ductile, heat-treatable). The combination is heat-treatable to higher strength, weldable (carefully), forgeable, castable, and machinable (slowly).
What Ti-6-4 does well:
- High strength-to-weight. 130 ksi UTS at 4.4 g/cm³ — yield-strength- per-weight beats every common steel and most aluminum alloys.
- Heat-treatable. STA condition adds ~20% strength over annealed for stressed components.
- Corrosion resistance. Nearly as good as CP Ti in most environments except hot-salt SCC at >290°C (an aerospace-specific concern).
- Cryogenic toughness. Retains ductility to LH₂ temperatures.
- Forging response. Fine alpha grain structure with controlled hot-work history; the standard alloy for forged aerospace parts.
What Ti-6-4 doesn’t do:
- Cold formability. Essentially nil. Form hot (200–650°C minimum) or use a more ductile alloy (CP Ti, Grade 9 Ti-3-2.5V).
- Welding without skill. Welds well with full inert shielding, but margin for error is small.
- High-temperature service above 400°C. Use Ti 6242, Ti 6246, or IMI 834 for higher service temperatures.
- Sliding wear. Galls like all titanium. Surface-treat or use dissimilar bearing materials.
Why Ti-6-4 dominates titanium use
The alloy was developed in the 1950s and turned out to hit a near- optimal balance for aerospace structural use: moderate density, high strength, good toughness, weldability, forgeability, machinability, and reasonable cost (relative to specialty Ti alloys). Aerospace specifications stabilized around Ti-6-4 in the 1960s, and the infrastructure (mills, forging dies, AMS specifications, test methods) built up around it makes alternative titanium alloys economically hard to justify for general structural use.
A few specialty alloys edge out Ti-6-4 in narrow regimes — Ti 6242 and 6246 for higher temperatures, Ti 5111 and 6Al-6V-2Sn for specific strength targets, Ti-15-3-3-3 for cold formability — but for the bulk of titanium structural applications, Grade 5 is the default and specialty alloys carry premium for narrow benefits.
Machining notes
Ti-6-4 is significantly harder to machine than CP Grade 2 — same low thermal conductivity (worse, actually: 6.7 W/m·K), higher strength, and more strain-hardening behavior at the cutting zone. Tool life is short and cutting parameters are conservative.
Production recipes:
Turning annealed Ti-6-4:
- Speed 30–50 SFM, feed 0.005–0.015 in/rev, DOC up to 0.10 in
- Sharp carbide, polished rake, 0° or slightly negative lead angle
- High-pressure (>1000 psi) through-spindle coolant — flood emulsion or synthetic, never chlorinated
- Rigid setup, minimize tool overhang
Turning STA Ti-6-4:
- Reduce speed 25–30%; feeds similar
- AlCrN or AlTiN coating extends tool life
- Tool life ~half of annealed material
Milling:
- Speed 25–40 SFM for end mills; carbide solid or insert
- Climb mill, full-flute engagement
- High-pressure coolant directed at cutter
- Don’t dwell — keep tool moving through retract paths
Drilling:
- Speed 15–30 SFM (slower than turning)
- Peck cycles to clear chips and limit heat buildup
- Cobalt HSS or solid carbide; through-tool coolant where possible
- Stub-length drills for rigidity
Machinability rating is ~22% of B1112 (free-cutting steel reference). Plan tool life at 15–30 minutes per insert at full production parameters. Roughing inserts last longer than finishing inserts.
Workholding: Ti-6-4’s low modulus (~half of steel) means clamping forces deflect the workpiece more than expected. Soft jaws, distributed contact, minimum effective clamping pressure. Long thin parts deflect under cutting forces; support fixtures or follower rests are common.
Chip handling: Pyrophoric. Wet collection, no dry conveyors, Class D dry powder fire suppression on hand. Treat Ti chips like flammable solid waste.
Heat treatment
The two main conditions you’ll specify:
Mill annealed (AMS 4911 / 4928 / 4967): Default supply condition. 705–790°C / 30 min–2 hr / air or furnace cool. Equiaxed alpha+beta microstructure. ~895 MPa UTS / ~828 MPa yield minimum, 10% elongation. Use for most non-stressed and moderately- stressed parts.
Solution treated and aged — STA (AMS 4965): Higher strength. Solution treat 913–954°C / 1 hr / water quench, then age 524–552°C / 4–8 hr / air cool. ~1172 MPa UTS / ~1103 MPa yield min, 8% elongation. Used for high-stress structural components where the strength gain justifies the heat treat cost.
Important constraints on STA:
- Maximum useful section thickness ~75 mm (3 in) for consistent through-thickness response — quench rate falls off in thicker sections
- The aging treatment must be precise; over-aging or under-aging miss the property targets
- STA parts should NOT be welded in the aged condition — the weld locally softens and re-aging finished assemblies is rarely practical
Beta anneal: For applications where fracture toughness matters more than fatigue (landing gear, pressure vessels). Above the beta transus (~995°C), slow cool, then re-anneal at 730°C. Higher toughness, lower fatigue strength.
Welding considerations
Ti-6-4 welds well with full inert-gas shielding but is more sensitive to contamination than CP Ti. The alpha+beta microstructure becomes locally martensitic (alpha-prime) on rapid cooling, reducing ductility in the weld and HAZ.
Process selection:
- TIG (GTAW) with argon shielding is standard for general structural welds
- EBW (electron beam) gives narrow HAZ, low distortion, near- parent properties; standard for engine and missile structural welds
- Laser welding similar to EBW, less expensive setup
- MIG (GMAW) less common, fair for thicker section welds
- Friction stir welding preserves substantial parent strength, emerging for aerospace assemblies
Practical requirements (same as CP Ti, more critical):
- Argon shielding gas (helium for higher heat input)
- Trail shield covering bead until cool below 400°C
- Back-purge on tube and pipe welds (argon flowing inside)
- Welding chambers for fatigue-critical work
- Cleanliness absolute — stainless brush dedicated to Ti only, acetone or MEK degrease (NEVER chlorinated solvents)
- ERTi-5 filler for matching properties; ERTi-23 (ELI) for high- purity / fatigue-critical service
Welded Ti-6-4 typically loses 10–20% UTS in the weld and HAZ vs parent metal even with optimal practice. Design weld zones with reduced allowable stress, or use mechanical fasteners for the most fatigue-critical joints. Post-weld stress relief (540–650°C / argon or vacuum) reduces residual stress and slightly recovers ductility.
Galling: still a problem
Ti-6-4 galls just like CP Ti. The TiO₂ passive film is thin, sliding contact breaches it, and exposed Ti-on-Ti or Ti-on-steel welds microscopically. Design rules:
- Surface-nitride sliding faces (TiN gives ~2000 HV, eliminates galling)
- For threaded fasteners, use silver-plated, MoS₂, or PTFE-based thread lubricants
- For bushings, use bronze or DU (PTFE-impregnated bronze) — not steel-on-Ti
- For taper junctions in orthopedic implants, surface-treat both faces; modular Ti hip systems use this approach
Applications by industry
- Aerospace — airframe fittings, brackets, bulkheads, landing gear, fastener systems, engine fan blades and discs (up to ~400°C section). The dominant aerospace structural titanium.
- Defense — missile structure, armor inserts, military aircraft components, naval propulsion shafts.
- Medical (non-implant) — surgical instruments, dental drills, external fixation hardware. Implants use Grade 23 ELI.
- Sporting goods — golf club heads (forged Ti-6-4 driver faces are this alloy), premium bicycle frames (welded tubing), titanium baseball bats.
- Motorsport — connecting rods, valve retainers, fasteners, exhaust components. Ti-6-4 STA is the standard high-stress motorsport alloy.
- Marine — high-strength propulsion shafts, deepwater housings, submarine internals. CP Ti for general marine; Ti-6-4 where strength matters.
- Oil and gas — subsea tieback connectors, drill pipe, downhole tools. STA condition for high-strength downhole hardware.
- Additive manufacturing — Ti-6-4 is the dominant AM titanium. Laser powder bed and electron beam printed parts are common for aerospace brackets and medical hardware. HIP post-processing recommended for fatigue-loaded parts.
- Cryogenic — pressure vessels, piping for LH₂/LOX service where high strength and chloride resistance are both required.
Failure modes worth designing around
Galling — universal Ti issue. Surface-treat any sliding contact.
Alpha-case from high-temperature air exposure. Heat-treat under vacuum or argon, or remove alpha case by acid pickling before service.
Hot-salt SCC — chloride salt deposits on Ti-6-4 above ~290°C cause stress corrosion cracking. Documented in jet-engine compressor parts contaminated by chloride-containing cleaning wipes. Use chloride-free cleaners on Ti aerospace hardware in service.
Fatigue at notches and surface defects. Ti-6-4 is notch-sensitive relative to steel. Generously radius all transitions, shot peen fatigue-critical surfaces, and polish-finish machined faces where fatigue matters. Ti-6-4 endurance limit is ~510–620 MPa for smooth specimens; notch factor can reduce this by half.
Hydrogen embrittlement from acid pickling residue or galvanic over-protection. Beta phase preferentially absorbs hydrogen, forming hydrides. AMS limits H₂ to 0.0125% in supplied material.
Brittle fracture from microstructure variations. Beta-annealed Ti-6-4 has lower ductility than alpha-beta annealed; specify heat treatment precisely per AMS callouts.
Methanol SCC — pure methanol + tensile stress = cracking. Same mechanism as CP Ti.
Pyrophoric chip hazard. Wet collection, Class D fire suppression on hand.
Welding strength reduction — typically 10–20% loss in weld and HAZ vs parent. Design accordingly or use mechanical fasteners.